The invention relates generally to process control and, more particularly, to an improved apparatus for tuning process control equipment according to the characteristics of the process itself.
"Process control" refers to the control of the operational parameters of a process by monitoring one or more of its characteristics over time. It is used to ensure that the quality and efficiency of a process do not vary substantially during a single run or over the course of several runs. Process control has application in both the manufacturing and service sectors. It has particular importance in the process control loop affecting product quality and the protection of personnel.
A process control unit, or "controller", typically operates by monitoring and comparing a process characteristic, the controlled variable, with a desired setpoint level to determine whether the process is operating within acceptable bounds. As the controlled variable begins to deviate from the setpoint, the controller manipulates one of the process input signals, the manipulated variable, to bring the process back to the desired level of activity.
Among the controllers developed by the art to minimize deviations in the controlled variable are the so-called proportional-integral-derivative (PID) controllers. These controllers generate the manipulated variable signal as a predetermined mathematical function of the controlled variable signal.
The output of a PID controller can be expressed by the mathematical relation: ##EQU1##
In this equation, m represents the controller-generated manipulated variable, which is input to the process itself to "manipulate" the process towards its desired setpoint range. The difference between the controlled variable c and the setpoint r is the error e. The variable P refers to the proportional band, usually expressed in percent, which establishes the range whereby a process deviation can occur without saturation. The time-rate derivative component, dc/dt, operates on the controlled variable with a time constant D. It is afterwards integrated by positively feeding back the controller output through a lag with a time constant I.
The serial action of the derivative component and feedback integration characterizes a PID controller. Quick process changes are, it is said, predicted by such controllers through the derivative time-rate component, dc/dt, inducing initial and large controller actions even though the actual deviation is small.
Another controller developed by the art is the proportional-integral (PI) controller. It is very similar to the PID controller but does not have the derivative component, i.e., the D parameter representing the derivative time constant. Without this derivative parameter D, the PI controller simply integrates process deviations over time and induces controller changes when the variation deviates in magnitude from the setpoint r.
Both the PID and PI controllers have acute limitations that depend upon the processes under control. For example, a PI controller is ineffective in controlling processes dominated by "lag", Lag is the time between an initial change in the controlled variable c due to change by the controller manipulated variable m and the time when the controlled variable c reaches 63.2 percent of its final value.
PID controllers fare better with lag-dominant processes because of the derivative-rate action, but its derivative-rate capability is useless if the process is predominantly "deadtime". Deadtime is the time it takes a change in the manipulated variable m applied to a process to be reflected by any change in the controlled variable c generated by that process.
For these reasons, the art developed deadtime controllers, which are constructed by adding a "deadtime" element, i.e., a time delay .tau..sub.d into the integral feedback loop of a typical PID or PI controller. This time delay, called the controller deadtime .tau..sub.d, improves a deadtime controller's performance over both the PI and PID controllers by reducing the size, area, and response time of a deviation caused by a load change.
But this improved performance comes at a price. The process control loop can go unstable, even permanently, if the deadtime controller parameters, e.g., P, I, D, and especially .tau..sub.d are set, or "tuned", incorrectly. The success of a deadtime controller, therefore, hinges even more critically on how well it can be tuned.
Deadtime controllers can be pretuned, according to prior art teachings, by modifying the controller parameters before operating with a process. But, ensuring that the process remains stable thereafter is difficult, especially if the controller is slightly mis-tuned. Deadtime controllers are particularly susceptible to a phase mismatch between the controller and process deadtimes.
It is, accordingly, an object of the present invention to provide improved systems for tuning deadtime process controllers during process control activities.
More particularly, an object is to provide improved methods and apparatus for tuning process control equipment.
Still another object is to provide improved deadtime process control systems that better maintain tuning with the processes they control.
These and other objects are evident in the description within.